The present invention generally relates to lighting systems and related technologies. More particularly, this invention relates to optical interference filters suitable for use with lighting systems, for example, halogen lamps.
Halogen filament tubes, also referred to as halogen IR (infrared) filament tubes (lamps), are often provided with a coating system on an outer surface of the tube for the purpose of spectrally reflecting IR radiation emitted by a filament within the tube. The coating system, referred to as an optical interference filter or more simply an interference filter, may reflect IR radiation back to the filament, where a portion of the reflected radiation can be absorbed to increase the efficacy of the filament tube, for example, by reducing the electrical power needed to operate the filament tube at a constant filament temperature. The interference filter is typically designed to transmit sufficient visible radiation to promote the lumen output and maintain the color of the light generated by the filament tube.
A nonlimiting example of a halogen filament tube 10 is represented in FIG. 1. The tube 10 includes an envelope (capsule) 12 formed of a light-transmissive material, nonlimiting examples of which are quartz (fused silica) or certain glass materials capable of withstanding high temperatures over extended periods of time. Two leads 14 enter the interior of the envelope 12 at one end of the tube 10, where they are electrically and mechanically attached to opposite ends of a filament 16 within the envelope 12. The filament 16 is typically composed of tungsten, carbon, or another electrically-conductive material. An optical interference filter 18 is indicated as being present as a coating on at least a portion of a surface of the envelope 12, typically the outer surface of the envelope 12.
As noted above, the filter 18 is intended to modify and/or enhance the energy efficiency of the filament tube 10. Suitable materials for the filter 18 typically exhibit the following characteristics: sufficient optical properties to be incorporated into a practical filter design; thermal stability at lamp operating temperatures (for example, 600° C. or higher) in an oxidizing or reducing environment in the envelope 12; chemical stability with the materials of the envelope 12 and other potential layers of the filter 18; and suitable thermal expansion properties in comparison to the materials of the envelope 12 and other potential layers of the filter 18. A known construction for the filter 18 is a multilayer coating system that includes alternating layers of relatively high and low refractive index materials that in combination promote the reflection of IR radiation (generally wavelengths of about 800-2500 nm) while transmitting visible radiation (generally wavelengths of about 400-750 nm). Various different coating compositions have been proposed, many utilizing silica (SiO2) as the lower refractive index material. As the higher refractive index material, various oxide compositions have been proposed, including oxides of hafnium, niobium, tantalum, titanium, zirconium, vanadium, cerium, and compounds thereof, for example, titanium oxide (titania; TiO2) alloyed (complexed) with oxides of zirconium, niobium, tantalum, etc. Two such examples are a high refractive index material containing about 90 mol % titania and about 10 mol % of a mixture of titania and niobia (Nb2O5) reported in U.S. Pat. No. 4,940,636, and a high refractive index material containing about 96-99 mol % titania and about 1-4 mol % niobia reported in WO 2007/0104462. In the binary phase diagram for the titania-niobia system, these levels generally provide for a titania solid solution containing a TiO2.Nb2O5 phase. However, film compositions containing high levels (for example, about 95 mol %) of titania are susceptible to undergoing a phase transformation at the relatively high operating temperatures of a halogen filament tube, and such phase transformations may be catastrophic to an interference filter. Another example reported in U.S. Pat. No. 4,940,636 is a 1:1 molar ratio of titania and niobia that yielded a single stoichiometric phase, TiO2:Nb2O5, also referred to as Nb2TiO7. Finally, WO 99/53526 reports a high refractive index material as containing 20-40 wt % niobia with the balance tantala (Ta2O5), which in molar percentage is about 29-53 mol % niobia. According to the teachings of WO 99/53526, films excessively high in niobia content are at risk of darkening over the life of a halogen lamp and that, to mitigate or abate film darkening, the filament tube may contain an oxidizing environment.
In view of the foregoing, there are limitations and disadvantages associated with existing materials used for optical interference filters of types that can be applied to halogen filament tubes, and there are ongoing efforts to develop improved interference filters for such high temperature applications.